1. Field of the Invention
The present invention relates to a diode and more specifically relates to a diode which realizes shortening of a reverse recovery time in an off state and reduction in leak current.
2. Description of the Related Art
FIG. 6 shows a cross-sectional view of a conventional pn junction diode 60.
In the pn junction diode 60, an n− type semiconductor layer (epitaxial layer) 52 is stacked on an n+ type silicon semiconductor substrate 51 and ions of p type impurities at high concentration are implanted and diffused into a surface of the n− type semiconductor layer 52 exposed from an insulating film 54 so as to provide a p type impurity region 53. In a peripheral edge of the substrate, an n type impurity region 56 and a metal layer 57 coming into contact therewith are provided. An anode electrode 55 is provided on a surface of the p type impurity region 53 and a cathode electrode 58 is provided on the entire back surface of the n+ type silicon semiconductor substrate 51. This technology is described for instance in Japanese Patent Application Publication No. Hei 10 (1998)-335679.
The pn junction diode with enhanced switching speed (Fast Recovery Diode: FRD) requires shortening of a reverse recovery time trr that is a time required for discharging an accumulated charge quantity Qrr.
As a method for shortening the reverse recovery time trr, it is conceivable to reduce an impurity concentration of the p type impurity region 53 and thus to reduce an amount of holes injected into the n− type semiconductor layer 52 to be a drift layer.
However, when the impurity concentration of the p type impurity region 53 is reduced, an amount of carriers (holes) accumulated in the n− type semiconductor layer 52 is reduced as a matter of course. This leads to reduction in a conductivity modulation effect. Therefore, there is a problem of an increased forward voltage VF near a rated current.
Moreover, there has been known a method of doping the n− type semiconductor layer 52 with heavy metal, a so-called life time killer. However, such method has a problem of deterioration in characteristics of the forward voltage VF due to increase in resistance when the layer is doped with too much heavy metal.
Moreover, in space charges, a leak current and a life time are in an inverse relationship. Therefore, when the life time is shortened by introduction of the life time killer, there is also a problem of an increased leak current.